Substations in high and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switching devices and power transformers, which are generally arranged in switch yards and/or bays. These primary devices are operated in an automated way via a Substation Automation (SA) system. The SA system includes secondary devices, among which Intelligent Electronic Devices (IEDs) are responsible for protection, control and monitoring of the primary devices. The secondary devices may be hierarchically assigned to a station level or a bay level of the SA system. The station level often includes a supervisory computer having an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and running a station-level Supervisory Control And Data Acquisition (SCADA) software, as well as a gateway that communicates the state of the substation to a Network Control Centre (NCC) and receives commands from it. IEDs on the bay level, which are also termed bay units or protection IEDs, are connected to each other as well as to the IEDs on the station level via an inter-bay or station bus primarily serving the purpose of exchanging commands and status information.
A communication standard for communication between the IEDs of a substation has been introduced by the International Electrotechnical Committee (IEC) as part of the standard IEC 61850 entitled “Communication Networks and Systems in Substations”. For non-time-critical report messages, section IEC 61850-8-1 specifies the Manufacturing Message Specification (MMS, ISO/IEC 9506) protocol based on a reduced Open Systems Interconnection (OSI) protocol stack built upon the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and upon Ethernet and/or RS-232C as physical media. For time-critical event-based messages, such as trip commands, section IEC 61850-8-1 specifies the Generic Object Oriented Substation Events (GOOSE) directly on the Ethernet link layer of the communication stack. For very fast periodically changing signals at the process level such as measured analog voltages or currents, section IEC 61850-9-2 specifies the Sampled Values (SV) protocol which also builds directly on the Ethernet link layer. Hence, part 9 of the standard defines a format to publish, as multicast messages on an industrial Ethernet, digitized measurement data from current or voltage sensors on the process level as a substitute to traditional copper wiring.
Hence, an IED controls actuators of assigned primary devices on the basis of signals from assigned sensors for switch or transformer tap changer position, temperature, voltage, current, etc. signals from other IEDs, and signals from the supervisory system. Conversely, an IED communicates a state or behavior of its assigned primary devices (e.g., selected sensor readings) to other IEDs or to the supervisory system. The signals can be transmitted as network messages, for instance according to IEC 61850-8-1 or IEC 61850-9-2 messages as introduced above.
Secondary devices on a process-level of the SA system include conventional Instrument Transformers (ITs) for voltage (Voltage Transformers (VT)) and current (Current Transformers (CT)) measurement, gas density or pressure sensors, as well as contact probes for sensing switch and transformer tap changer positions. Furthermore, exemplary intelligent sensors such as non-conventional electronic or optical sensors for current or voltage include an Analog to Digital (A/D) converter for sampling of analog signals, and are connected to the bay units via a dedicated bus as part of an intelligent process interface. The latter replaces the conventional hard-wired process interface that connects conventional ITs in the switchyard, via dedicated copper wires and junction boards, to different bay units which individually sample the analog signals from the ITs.
Additional secondary devices, so-called “protection relay devices” or “conventional IEDs”, are connected to the ITs as well as to switching devices like Circuit Breakers or Disconnectors, via dedicated cables or wires, for monitoring status and/or position of the switching devices and for issuing commands to close or open the switching devices based on decisions from corresponding protection functions and control functions. It is known to use interposing or intermediate relays arranged between these devices and an actuator or tripping/control circuit of a switching device to isolate the secondary from the primary devices. The interposing relay may also fulfill local or remote controlled blocking functions, an anti-pumping function, a trip circuit supervision function and a compressor pressure low function. The interposing relay can be integrated into the protection relay device or the conventional IED, or can be distributed fixed on the front or rear of a cubicle including the secondary devices.
For the purpose of synchronization with other internal data, sampling of analog signals by an A/D converter of a process interface (in, for example, electronic or non-conventional current and voltage sensors having a digital output, merging units, and bay units) requires correct time stamping. In other words, the devices that sample analog values and send the digitized messages to the network are synchronized to each other through a common source. This is today accomplished typically through a pulse-per-second signal coming from a GPS receiver, or through Ethernet-based protocols such as IEEE1588 guaranteeing accuracy levels which are suitable for those types of applications.
IEC 61850 defines several profiles for 9-2, a so-called full profile and a light edition (9-2LE). Common to both profiles is the main characteristics on how sampling data is encoded, time stamped and transmitted over an Ethernet-based network. The sending frequency is dependent on the electrical network line frequency or period (50 Hz or 60 Hz) itself. Data sampled 80 times per period is sent off the SA communication network in messages or packets at a rate of 4 kHz or 4.8 kHz for protection and control purposes, while 256 samples/period for revenue metering and power quality monitoring applications result in a rate of 12.8 kHz or 15.36 kHz, respectively.
Depending on their respective design, different conventional ITs for current and voltage have different characteristics, for example, with respect to accuracy, phase error, transient behavior, frequency response and saturation. Furthermore, non-conventional sensors including, for example, voltage transducers based on the electro-optic effect (Pockels effect) in a crystal and current transducers using a Rogowski coil or the magneto-optic effect (Faraday effect) in bulk glass or in an optical fiber likewise entail different characteristics. Different IT types are designated TPS, TPX, TPY, and TPZ, while the transformer type TPE has been introduced to designate electronic transformers.
U.S. Pat. No. 6,845,301 discloses a so-called Merging Unit (MU) as part of an intelligent process interface. The MU may be connected, via dedicated copper wires or via a dedicated serial bus, to a plurality of sensors. The MU is configured to process different signals indicative of current or voltage in a substation of an electric power system, by time-synchronizing the signals and by generating and transmitting, over a substation communication network a network message including sampled values of the signals.
EP 1898509 discloses a protection and control system for electric power networks with signal and command (process) interfaces termed “bricks” and located at the primary equipment. Each brick converts analog input signals into digital representations and can execute commands generated by a protection and control IED. Each brick is independently connected via dedicated point-to-point optical links to a plurality of IEDs, thereby explicitly excluding the use of Ethernet switches to share data among IEDs.
U.S. Pat. No. 7,180,729 discloses an arrangement for controlling and monitoring a switching installation in which a station control computer is configured to perform protection and control functions. Furthermore, the arrangement includes switch control devices which are used to digitally control switches of the switching installation and which include digital inputs and outputs. The arrangement also includes a separate electronic conversion system provided with digital outputs. The digital inputs and outputs of the switch control devices and the digital outputs of the electronic conversation system are logically connected to the station control computer by means of a communication connection or bus.